The design of a hydro-pneumatic spring suspension presents a particular challenge, particularly in agricultural or industrial work machines, such as tractors, because in such applications extreme variations in axle load are possible. The axle load bandwidth usually exceeds the pressure ratio possible with membrane-type pressure accumulators. Therefore, in order to maintain the operating range of the pressure accumulators, the system is preloaded. This preload operates as an additional axle load, whereby the ratio of minimum to maximum axle load can be reduced.
The preload of the system is rarely performed by a mechanical preload, with which a single-acting cylinder can also be applied. Instead a double-acting cylinder is typically applied in which the rod end pressure chamber is preloaded to a certain pressure over a pressure accumulator.
The pressure applied to the side of the rod end chamber may correspond to the maximum system pressure as disclosed in German Patent Application DE-A43 08 460 or the pressure is adjusted by a pressure regulator to a certain pre-selected value as disclosed in German Patent DE-C-42 42 448. As in the case of every hydro-pneumatic system, both systems are provided with a spring rate that varies generally as a function of the square of the axle load. On the other hand, for a constant natural frequency of the chassis, a linear relationship would be ideal.
In order to approach a more linear relationship in practice it has been proposed in German Patent Applications DE-A-197 19 076 and DE-A-197 19 077 that a hydraulically controlled pressure control valve be provided in the rod end chamber supply line, whose control spring force is adjusted on the basis of the pressure in the supply line of the cylinder chamber. Thereby the pressure in the rod end chamber is controlled as a function of the pressure in the cylinder chamber of the hydraulic cylinder, where at low loads the additional load in the rod end chamber is a maximum and is steadily reduced to a minimal value at higher loads. Among other items the operating comfort should thereby be improved.
German Patent Application DE-A-41 20 758 describes a hydro-pneumatic vehicle spring suspension of the aforementioned type, in which a hydraulically actuated control valve is arranged in the supply line to the rod end chamber for wide spreads of axle loading, which is exposed on the one hand to the pressure in the rod end chamber and on the other hand to the pressure in the cylinder chamber, so that the pressure in the rod end chamber is controlled as a function of the pressure in the cylinder chamber. Thereby a higher spring stiffness should be attainable in the area of lower axle loads.
Specific axle loads can be generated by various ballasting conditions on the vehicle. For example, in order to equalize the weight of a heavy plow and to bring an adequate load upon the front axle, ballasting weights can be attached to the front of the tractor. This ballasting can result in the same axle loading on the front axle as a tractor that is equipped neither with a plow nor with front weights. However, with previously known vehicle spring suspensions the spring stiffness cannot react to differing ballasting conditions or operating or driving conditions of the vehicle, that is, the spring stiffness is independent of these conditions and is a function only of the immediate axle load. This can lead to the fact that a heavily ballasted tractor is sprung too softly with regard to the pitching natural frequency, while an un-ballasted tractor is too stiff.
Accordingly, there is a clear need in the art for a hydro-pneumatic spring suspension arrangement of the type noted initially through which the aforementioned problems are overcome. In particular the given invariable square relationship of the spring rate to the axle loading should be countered and the spring rate should be made to conform to ballasting conditions and operating or driving conditions of the vehicle and not depend entirely on the actual axle loading.